The present invention relates to a power supply unit suited to lighting a light source, and to a method of controlling the same.
A power supply unit (or “stabilizer” or “ballast”) that uses a back converter (a step-down converter) or the like is known as a power supplying apparatus for lighting a discharge lamp. Discharge lamp such as a high-pressure discharge lamp (or “high-intensity discharge” (HID) lamp) is one of types of light sources. Discharge lamps are used in a variety of applications. One application is the light source (provided as an illumination unit) of an image output system, with the concept of an “image output system” including a projector (a front projector or a rear projector) that displays color images on an image forming medium such as a screen. One method of displaying color images is to display pixels of multiple colors (for example, the primary colors R, G, and B) using time division. One means for producing light of the respective colors from light (a light beam) outputted by a light source for white or near white light according to time division uses a rotating color filter called a “color wheel”. By modulating the light of the respective colors produced by the color filter according to time division in synchronization with the movement of the color filter, it is possible to form images of the respective colors. One example of a device that generates images by modulating light (variously referred to as a “light modulating device”, a “light valve”, or an “image generating device”) is an image generating device that includes a plurality of optical elements that can modulate light in pixel units. Examples of this type of image generating device are both transmissive and reflective liquid crystal devices and DMDs (digital mirror devices).
Japanese Laid-Open Patent Publication No. 2006-349912 discloses a projection system that is lighted using a DC current and includes a rotating color filter that is divided into a plurality of color segments that are red, green, and blue or a plurality of segments that includes segments for blue, green, red and others so as to subject a beam emitted from a light source to time division. This projector system generates synchronization signals with a plurality of ON/OFF patterns at a predetermined time in synchronization with optical elements and the rotation of the color filter. Japanese Laid-Open Patent Publication No. 2006-349912 also discloses a construction that realizes superior tonal reproduction across the entire tonal range by superimposing a current, such as “first negative pulses”, of a certain current intensity in accordance with the ON/OFF patterns onto the applied DC current a fixed time (T0) after the timing of a falling edge corresponding to a red segment or a blue segment.
Japanese Laid-Open Patent Publication No. H02-119005 discloses an illumination device for an electronic endoscope where illumination light from a light source is emitted via a rotating color wheel including a plurality of filter areas. In this publication, the illumination device is also equipped with a position detecting means for detecting the respective filter areas provided on the rotating color wheel and a light source intensity adjusting means that adjusts the intensity of light emitted from the light source in accordance with the respective filter areas based on a signal from the position detecting means.
Japanese Laid-Open Patent Publication No. 2006-227440 discloses a technology that is operable, when light of a plurality of colors such as RGB is repeatedly generated using a filter from light generated by an AC lamp for use in a projector device, to synchronize the timing of changes in the polarity of the lamp current (i.e., the AC current supplied to the AC lamp) with the timing at which the color of light changes. This publication states that by changing the magnitude of the lamp current in half cycles synchronized with the cycle for generating light of a specified color, the intensity of the light of that color can be increased and decreased. This publication also states that it is possible to adjust the intensity of light of any color and therefore possible to freely adjust the white balance of projected images projected by the projector device. To do so, power control signals are used in addition to synchronization signals.
There is demand for flexible control that can synchronize the output (intensity) of an illumination device with movement of a color filter (color wheel, etc.), an image signal, or the like. One method of controlling the output of an illumination device is to control the power supplied to the light source, for example the discharge lamp, of the illumination device. Since the voltage supplied from the power supply unit to the discharge lamp becomes substantially constant when discharge is stabilized, the supplied power is controlled by controlling the current supplied from the power supply unit to the discharge lamp. Accordingly, there is demand for a power supply unit that can easily change the value of the supply current in synchronization with a synchronization signal.
A power supply unit that can flexibly change the supply power in synchronization with predetermined signals has wide applicability for the various specifications of illumination devices. This makes it possible to reduce the time required to develop and design a power supply unit, which reduces costs. For example, to improve contrast, some color filters include segments that are white (openings, W) or another color in addition to segments of the colors red (R), green (G), and blue (B). The respective areas of the segments of a color filter differ according to how the illumination device is being applied, and the light intensity required by different segments often changes between illumination devices. In addition, when a sequence of segments of different colors such as RGB is treated as a unit, the cycle for controlling the light intensity does not need to match such unit, and the light intensity may be controlled with a plurality of such units as one cycle. The light intensity may be changed in synchronization with the segments of the respective colors on the color filter, or the light intensity may be changed in between such segments.
A power supply unit that can flexibly (dynamically) change (reconfigure) the power supplied in synchronization with predetermined signals during operation of an illumination device and/or during initial settings enables the illumination device to be put to a wider range of applications. For example, such power supply unit makes it possible to equip an illumination device with a function for changing the white balance and/or tones (combinations of colors) in accordance with the type of images displayed by the user. By doing so, it becomes possible to reduce the processing load of an image generating device and to display images of significantly higher quality.